Ultrasound probes for medical imaging support typically up to 90% relative bandwidth. In ultrasound imaging the bandwidth in absolute terms is directly proportional to the achievable range resolution, whereas the ultrasound center frequency is inversely related to the achievable imaging range. High relative bandwidth is therefore desirable to obtain high range resolution at good image range. To image the human hearts of adults, children, and small babies, for example, it is often necessary to have three different probes with typical center frequencies of 2.5 Mhz, 5 Mhz, and 9 Mhz. With wider band probes, the range needed for these applications may be covered with two probes and, in addition, have more flexibility to optimize the transmit signal and the receive filtering to the requirements encountered in each imaging situation.
A limitation in the use of Doppler for measuring blood velocity and/or motion of tissue is often detection sensitivity, whereas the spatial resolution required for good tissue imaging is of less importance. One may wish to have the Doppler centered at, for example, 2 Mhz, whereas the imaging could be at frequencies ranging up to 5 Mhz, which can only be accomplished with probe bandwidths exceeding today""s standard, or with probes showing two passbands: one used for Doppler measurements, and the other for imaging.
Detection of second harmonic signals generated in tissue is used by most ultrasound scanners on the market today. The second harmonic ultrasound beam is more narrow with suppressed sidelobes compared to the fundamental frequency beam. By enhancing the second harmonic signal and suppressing the fundamental in the receiver, the image quality is significantly improved. For optimum performance, the ultrasound transducer should be able to cover the fundamental frequency and the second harmonic frequency, both with high bandwidths.
It has been proposed by researchers that detection of ultrasound contrast agents could be significantly enhanced by using the strong nonlinear generation of the reflected ultrasound from contrast agents. The differentiation of contrast agent from tissue would be significantly improved by detecting the 3rd and 4th harmonic of the fundamental ultrasound frequency.
The above ultrasound applications require transducer technology which are either extremely wideband, or which can manage different frequencies in the transmit and the receive mode.
Current probe technologies are limited to bandwidths ranging from about 70% to 90%. In harmonic imaging, for example, the center frequency of the transmit pulse is positioned on the low frequency side of the transducer pass band. The second harmonic will then be positioned on the upper side of the probe pass band. The result is that the transmit pulse and the received harmonic pulse are both distorted in the transducer. The distortion leads to extended pulse duration with loss of range resolution in the image. Moreover, the distortion also causes irrecoverable loss of imaging sensitivity. The manufacturing variability in probe center frequency and bandwidth leads to variations in image quality, and to variation in insertion loss of the second harmonic detection. There is today no probe technology commercially available for the detection of 3rd and 4th harmonic frequencies.
Another important issue in harmonic enhanced image quality is the need to suppress harmonic frequencies in the transmit signal. The image quality advantage is obtained almost solely from harmonics generated in tissue. Transmitted harmonic signals will have more or less the same sidelobes as the fundamental frequencies, and the advantage to the image quality of a narrow beam is lost. Thus there is a need for a transducer and transmitter that are designed together to obtain transmit pulses which are optimum for harmonic imaging.
The reflected signal from tissue contains frequencies from the fundamental and from the harmonics. Near the probe, the harmonic has not been able to develop, and most of the energy is on the fundamental frequency. Deeper into the body (typically beyond one centimeter or so) the harmonic signal has developed to provide a strong echo which can be extracted from the fundamental in the receiver to give the image quality improvements. At large depths, the second harmonic is more heavily attenuated than the fundamental, and most of the reflected energy is on the fundamental frequency. Since the ratio of harmonic to fundamental power increases with the output transmit power, the range where the harmonic is effective for imaging will also increase with the output transmit power. The mix of energy in different frequency bands thus varies with the fundamental frequency, the transmit pulse power, the attenuation in tissue, and the imaging depth range. Therefore, the signal received back is so greatly modified that a simple element design is typically not optimum both for transmission and reception at all depths.
One approach to harmonic imaging is described by de Jong et al. (2000 IEEE Ultrasonics Symposium 1869-1876). De Jong et al. made a dual-frequency array transducer. The transducer comprised two types of transducer elements, the two types of elements having different center frequencies. The transducer had 48 of each kind of element, the elements being interleaved with adjacent side-by-side elements having a different center frequency. The De Jong et al. configuration leads to a large transducer footprint, which is a disadvantage in most applications.
Another conventional device comprises a stack of two transducers on top of each other. The transducers are tightly coupled acoustically because they are laminated into a sandwich. Each transducer is connected to either an instrument transmitter or a receiver but not to both a transmitter and a receiver.
An embodiment of the present invention comprises a transducer element for ultrasound transmission and reception. A first active transducer layer is connected to a first receiver and a first transmitter. A second active transducer layer is laminated to the first active transducer layer and is connected to a second receiver and a second transmitter. The transducer element may comprise passive circuitry wherein a first pulse and a second pulse are processed by the passive circuitry so as to have one or more of the following properties prior to being combined into a single ultrasound pulse: different amplitudes, different time delays, and different shapes. Each active layer may be connected to a separate voltage source when the transducer element is in transmit mode. The transducer element may comprise a switch for switching the transducer element from transmit mode to receive mode. The tuning circuitries applied to the transducer layers in transmit may depend upon the mode of operations, and may for example be different for imaging, Doppler, and color flow. Likewise the tuning circuitries applied to the transducer layers during reception may be different from those applied in transmit, and the tuning circuitries may be different in different modes of operation. Switches may be used to facilitate connection to the appropriate tuning circuitries.
Another embodiment of the present invention comprises a plurality of transducer elements configured into an array to provide electronic beam steering and focusing. The various elements may be arbitrarily located with respect to each other. However, in the most common structures the transducer elements are organized in a row to facilitate beam steering and focusing in an arbitrary direction within a sector in a plane, or the transducer elements may be organized in a matrix to facilitate beam steering and focusing into a volume. The various elements may have different surface area, and/or different first and second active laminated transducer layer materials and/or different thicknesses. The first and second active laminated transducer layer of each element may be connected to separate independently programmable transmitters and receivers with separate switchable tuning circuitry. The various transmitters can be programmed to deliver signals with different amplitudes, different time delays, and different envelopes prior to becoming combined into a single acoustic transmit signal which is better optimized for imaging across the complete imaging scenario. The transducer array may comprise passive circuitry to reduce non-fundamental frequencies in the ultrasound pulse produced by combining the signals from the first and second transmitters. The transducer array may process signals from each active transducer layer separately to provide simultaneous display of a region with different receive beam forming and filtering. The transducer array may combine filtered and amplified receive signals from each active transducer layer before display to enhance image quality. The transducer array may combine filtered and amplified receive signals from each active transducer layer before display to display specific signal features. The transducer array may combine filtered and amplified receive signals from each active transducer layer before display to display harmonic frequency generation and imaging of contrast agents within tissue.
Still further aspects of the invention include a method of transmitting an ultrasound pulse comprising the step of providing a transducer element having a first active transducer layer connected to a first transmitter and a second active transducer layer laminated to the first active transducer layer and connected to a second transmitter. Other steps include transmitting a first signal into the first active transducer layer and transmitting a second signal into the second active transducer layer.
Another aspect of the present invention is a method of transmitting and receiving an ultrasound pulse. The method may include the step of providing a transducer array comprising a plurality of transducer elements wherein at least two transducer elements comprise a first active transducer layer connected to a first receiver and a second active transducer layer laminated to the first active transducer layer and connected to a second receiver. Additional steps may include transmitting a first signal into the first active transducer layer of at least two transducer elements, transmitting a second signal into the second active transducer layer of at least two transducer elements, wherein the first and second active layers of the transducer elements transmit an ultrasound pulse, filtering and amplifying a first signal in the first receiver of at least two transducer elements, filtering and combining the signals to form a first focused signal, filtering and amplifying a second signal in the second receiver of at least two transducer elements, filtering and combining the signals to form a second focused signal, and then optionally additional filtering of the two focused signals before the signals are combined into a final focused signal.